Magnetic resonance imaging at ultra-low fields
(ULF MRI) uses SQUIDs (superconducting quantum interference devices) to
measure spin precession at a microtesla-range magnetic field after
sample magnetization is enhanced by a stronger pre-polarizing field.
Here, the first ULF images of the human head acquired at 46 microtesla
measurement field with pre-polarization at 30 mT are reported. The
imaging was performed with 3 mm x 3 mm x 6 mm resolution using the
7-channel SQUID system designed for both ULF MRI and
magnetoencephalography (MEG). This result demonstrates feasibility and
potential of human brain imaging at microtesla magnetic fields.

A new mixing and filtering technique for
magnetic signal detection based on the principles of magnetic force
microscopy (MFM) is described. MFM adapts sensitivity of MFM to
force-detect RF-magnetic signals at distances necessary for in-vivo
imaging. Instead of using a coil antenna with resonant LC network, an
untuned coil antenna is coupled magnetically to an ultra-sensitive, MFM
cantilever with an integral coil. Forces on the cantilever tip from the
untuned coil antenna cause deflection of the cantilever, whose
instantaneous position is measured by reflection of a laser onto a
detector. The resonant cantilever acts as an electromechanical filter
and sensitive signal transducer. The system is frequency agile, allowing
broadband operation and though thermal-noise limited, the system
provides several potential advantages over conventional NMR detector
coils; the parameters affecting the sensitivity of the antenna and their
effects on signal-to-noise ratio are calculated.

A conceptually new approach to enhance MRI
contrast by manipulating the intrinsic spin dynamics in the presence of
nonlinear feedback interactions has recently been demonstrated. In this
work, we fabricate an active RF feedback device to amplify and control
the radiation damping feedback field. To validate the efficacy of active
RF feedback, tumor detection and characterization in in vivo mice
injected with human lung cancers was investigated. It is shown that
active RF feedback circuit enables improved differentiation of
neighboring normal and abnormal tissues at low fields using conventional
probes/receiver coils.

11:06

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NMR Experiments Using No RF Coil: RF-Coilless NMR

Xiaoliang Zhang1, 2

1University of Minnesota,
Minneapolis, USA; 2University of California San Francisco,
San Francisco, USA

In this work, we investigate an unusual NMR
experiment method which uses no RF coils. The method is based on the
dielectric resonance (or sample self-resonance) property of a NMR sample
with finite boundaries. The NMR images and proton spectra of a
cylindrical water sample acquired on a 7T scanner are presented. NMR
with no RF coil may provide a more sensitive and simplified way to
perform NMR because an entire subsystem in the NMR signal receiving
chain, RF coil, is eliminated.

Magnetic Field Monitoring (MFM) system based on
NMR probes promises an effective method to correct B0 and gradient
imperfections in MRI. This is of great interest, because the practical
applicability of several advanced pulse sequences is hindered due to
these non-idealities. However, receive-only NMR probes are relatively
cumbersome to use, as the probes have to be aligned with the excitation
plane of the imaged object itself. In this paper, a more practicable
MFM setup is described in form of transmit-receive NMR probes. Having a
second exciter channel available, MFM monitoring can be flexibly
performed almost independent of the pulse sequence chosen.

1National Research Council
Institute for Biodiagnostics, Winnipeg, Canada

Primarily for use with transmit/receive
high-field phased arrays, the concept of distributing multiple,
non-magnetic Cartesian feedback transceivers around the back of the
imaging magnet is presented. Close proximity to the array coils ensures
minimal cable RF power loss, while the feedback corrects distortion and
allows inexpensive, non-magnetic transistor RF power amplifiers to be
used. The feedback also ensures that induced currents in coupled coils
(transmit and receive) are reduced by a factor of 100, and stable
performance has been verified with 2 coupled coils attached to two
instruments. Control is via a USB-based link to an external computer.

The link budgets of analog and digital wireless
transmission for MR signals have been made. Although analog transmission
may be has higher SNR and sensitivity in theory, the digital one is more
feasible with better stability and noise immunity. Also with the rapid
growth of wireless LAN (WLAN) for the data rates from 2Mbps up to
108Mbps and precipitous drops in prices of WLAN products, an application
of digital wireless transmission for MR signals based on 802.11b has
been realized.

11:54

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[Not Available]

Single Optical Fiber Transmission for Multi-Channel
MRI Using FDM Method

Gary X. Shen1,
Juan Wei, Jing Yuan, Yong Pang

1The University of Hong Kong,
Hong Kong, Hong Kong

A single analog optical link system for
multiple channel RF transmission is designed and implemented. All the RF
channel signals can be down converted to the different low frequencies
using frequency division multiplexing (FDM) method, and combined to a
single optical link for transmission. The primary bench tests show that
the system has very good dynamic range, linearity and SNR. This design
can significantly reduce the system complexity and cut the cost of using
multiple independent optical transmit links and optical receivers for
multi-channel array MRI applications.

In this work a multi-channel transmit extension
for a broadband RF electronics with the potential of driving large
numbers of transmit channels was developed. This novel electronics setup
was implemented on a 9.4 T animal scanner with 8 transmit and receive
channels and its functionality was successfully verified in experiments
of B1-shimming and Parallel Excitation.

A TEM transmission line coil with all port
drive was reported in the October 2007 issue of MRM. In this work, we
describe the integration of this deisign with a state of the art,
multi-channel all digital T/R chain. The receiver in the chain is a
Mercury Visage direct digital receiver which uses a fold-back Nyquist
digitization scheme in conjunction with analog anti-aliasing filters.
The transmitter is an experimental prototype based on ultra high speed
D/A converters controlled by field programmable gate array (FPGA)
chips. Phase, frequency, receiver gain, and transmitter amplitud are
all software controlled. The need for quadrature hybrids and/or power
dividers is neatly eliminated. The system is portable, and maybe used
at any filed strength between 0.2T and 11T with no special
modifications.